Programming chords in keyboard mode

Summary
At the moment the rows with the mute/launch and audition/section buttons have no function. It could be handy if a user could assign chords at this section which are played back when a button is pressed.

Minimal viable product
- press and hold at least one key in keyboard mode
- press one of the audition/launch buttons
- on release, the pressed keys are stored
- to play the chord back, press the assigned button

Additions
- The arpeggiator could also work with these assigned buttons